I have a 6gb iPod mini, and I like it a lot. It is only $50 more for 20gb, but I don't think I'd every really need 20 gigs. 6gb is enough to never hear the same song twice unless you're playing continuously for days, but it's small enough that I'm only sticking on my favorite playlists/songs.

If you want to have every single song from your 100 cds available at your fingertips, then go for the 20gb model. If you want to narrow it down to your favorites, the 6gb mini is plenty.